Most devices are capable of sensing or reproducing only a portion of the full range of colors that can be realized with monochromatic lights and discerned by a human observer. In the art, the term “gamut” refers to the range of colors that a device, such as a CRT or color printer, can produce.
The gamut for a particular device is determined by a variety of factors and will differ from one device to another. For example, CRT monitors use additive color reproduction to produce colors within their gamut. Each pixel within a monitor comprises three phosphors—red, green, and blue. These phosphors can emit light of varying intensities, depending on the excitation of the phosphor. The size of the phosphors is such that it is below the resolving power of the human visual system. Thus, the light emitted by individual phosphors merges with the light coming from its neighbors. Due to the phosphors having high chroma, the gamut of a typical CRT monitor is quite large compared to many other devices.
In contrast to CRT monitors, color printers use subtractive color reproduction wherein colorants (e.g., inks, toners, etc.) with various spectral characteristics absorb different parts of the light illuminating the print. In general, the intensity of the colors used in a color printer cannot be varied directly. Instead, a color printer creates perceived changes of intensity by printing dots of different sizes, which are below the resolving power of the human eye. As the human eye cannot resolve individual dots, the stimuli they produce blend together to produce the different colors of the printer gamut.
It is often desired to reproduce an image generated on one medium on a different medium. For example, it is often desired to reproduce an image generated by a CRT or scanner on a print medium. It is likely that many colors in the CRT gamut are outside the gamut of the printer. Colors outside the reproduction range of an output device are referred to as “out of gamut” colors. These “out of gamut” colors cannot be reproduced exactly. Instead, some “in gamut” color that is within the reproduction range of the printer must be substituted for each “out of gamut” color in order to reproduce the image. This substitution of colors is performed by gamut mapping algorithms.
Numerous techniques exist for mapping colors from an input gamut to an output gamut depending on the objectives sought. Some of the more common approaches include maintaining hue and lightness for the out-of-gamut colors and clipping chroma to the gamut boundary, or compressing the input gamut so that the colors fall within the output gamut. The chroma clipping and compression techniques work well for certain images, such as photographs, that contain few out-of-gamut colors. However, highly chromatic colors are very appealing for some images, such as presentation graphics. For these types of images, the chroma clipping and compression techniques may not produce acceptable results because the output images will be noticeably lower in chroma.
The purpose of using colors in images such as business graphics is to make information maximally appealing and discernable to the human eye. When a business graphics image was created, it was usually designed with the assist of a display device such as a CRT screen, which has a relatively large gamut. The designer of the image may not be able to foresee what type of medium that the created image will be reproduced on and what viewing conditions will be used. As a result, the design intention may not be fulfilled due to significant gamut differences between the CRT used and the medium that the image is to be reproduced on. Non-ideal viewing conditions can also significantly reduce the actual gamut of the reproduction.
Although it is always desirable to maintain reasonable color accuracy, including chroma accuracy, there are reasons that can justify the use of chroma enhanced colors for illustration purposes as stated above. One is that the human visual system is known to be highly adaptive to stimuli strength including color chroma and lightness. Color accuracy in chroma and lightness is therefore difficult to define for a business graphic image. In contrast, if a gamut mapping algorithm maps the colors to corresponding colors of maximum chroma while maintaining the contrast, it ultimately fulfills the initial intention of the designer of the image, which is to make information maximally appealing and discernable to the human eye. Also, when a reproduction device has some portion of gamut that is larger than the initial CRT gamut used for designing the image, such algorithms allow these colors to be used in the image. Further, in situations that the reproduction system has an unusually small gamut, such as some special label printers, such gamut mapping algorithm can make a vast visual difference in prints.
Consequently, there is a need for a gamut mapping algorithm especially adapted for use in business and presentation graphics or similar images with highly chromatic colors.